Internal combustion engines use fuel injectors to precisely control the amount of fuel inducted into the engine's cylinders. Also, fuel injectors atomize the liquid fuel, increasing the homogeneity of the air and fuel mixture. Air assist injectors are devices that use airflow to enhance the atomization of fuel injected into an engine's cylinder. Air assist injectors may be required on some vehicles where additional fuel vaporization can promote better combustion and lower regulated emissions. Further, the air flowing through the air assist injector can be controlled to maximize the benefit as a function of engine operating conditions.
For example, at low engine operating temperature, fuel atomization can be enhanced by controlling a flow valve to be substantially open, thereby allowing a large airflow through the air assist injectors. On the other hand, air assist injector airflow can be restricted by controlling the flow valve to be substantially closed as the temperature increases to prevent overrunning of the engine. Another example, which can be combined with the previous example, is that the flow valve can be controlled to open proportionally to an engine load, thereby allowing an increasing airflow through the air assist injectors as engine load increases to counteract the decreasing pressure ratio caused by increasing manifold pressure. Such a system is disclosed in U.S. Pat. No. 5,460,148.
The inventor herein has recognized numerous disadvantages with the above approaches. For example, when the engine is operating at a mid-load condition, the airflow through the air assist injectors will not be maximized because the flow valve will be partially restricting the airflow to the air assist injectors. This is due to the valve being open proportionally to engine load. In other words, at mid-load, the valve is not fully open. Further, when the engine is started at a warm temperature, the flow valve will be restricting the airflow, thus preventing optimal atomization of the injected fuel, which can cause less than optimal emission control. Another disadvantage is related to the reliance on the operating temperature of the engine. For example, as an engine ages, less torque, and thus less air, may be required to maintain a given speed at a given temperature due to decreased friction. Thus, if temperature alone is used, the engine may develop an overrunning condition due to excess air flowing through the air assist injectors, which is utilized in combustion.